Maleic anhydride, maleic acid, fumaric acid, and itaconic acid are used on a commercial scale for the synthesis of (a) unsaturated polyesters and (b) monoesters and diesters with monobasic fatty alcohols.
Group (a) products can be synthesized so that they contain terminal hydroxy groups. Such hydroxy terminated unsaturated polyesters are capable of polymerizing with acrylic and vinyl monomers, and of reacting with amino resins. Their reaction with amino resins leads to crosslinking under the influence of heat. Therefore, such polyesters can form thermosetting compositions. On the other hand, the group (a) polyesters are polymeric with regard to their unsaturation (that is, many double bonds per molecule) and, therefore, their copolymerization with other monomers, such as acrylic esters, and vinyl acetate, leads to rapid crosslinking and gellation.
Consequently, copolymerization of polymeric unsaturated polyesters with other unsaturated monomers, such as acrylic and methacrylic esters, and vinyl acetate, cannot apparently yield commercially useful solution or emulsion copolymers.
Group (b) monoesters and diesters are used as comonomers in solution, and in emulsion copolymers. However, they cannot impart hydroxyfunctionality and therefore crosslink ability, to any of their copolymers.
The compounds produced by the process of this invention are a variant of the group (b) diesters. The produced compounds are diesters prepared from an ethylenically unsaturated acid, typically maleic acid or anhydride, a primary alcohol, and a polyol. They therefore contain at least one reactive hydroxyl group in addition to the ethylenic bond.
Attempts have been made hitherto to prepare these compounds, since they should exhibit properties making them useful in copolymer systems with acrylic and other olefinically unsaturated monomers. None of these methods have proved to be commercially viable.
The most studied route is essentially a two step sequence. In a first step, a monoester, sometimes referred to as a half ester, of the ethylenically unsaturated diacid is prepared. This monoester is then reacted with an alkylene oxide, usually ethylene oxide or propylene oxide, (see U.S. Pat. Nos. 3,270,088; 3,399,229; 3,360,544; 3,481,973 and 3,494,605). This procedure has been found by us to have certain disadvantages. The first one is that it is limited to producing compounds where the hydroxyl group is beta to the ester linkage: this follows as a direct consequence of the opening of the 3-member oxide ring. This process is also not amenable to producing compounds containing more than one hydroxyl group. But what is perhaps most important these processes require as catalysts quite powerful compounds (e.g. U.S. Pat. Nos. 3,399,229 uses chromium trichloride; 3,481,973 uses pyridine, lutidine and other strong bases; 3,270,088 uses triethylamine) which cannot be readily removed from the product, as is some times admitted, for example the statement that the "product is only weakly green colored" in U.S. Pat. No. 3,399,229. It is our belief that the presence of these catalyst residues has an extremely adverse effect on the reaction products obtained. We have failed to prepare adequate copolymer systems from such products.
The attraction of the alkylene oxide route is that not until the oxide ring is opened is there a full hydroxyl group: therefore this method should minimize polyester formation. An alternative approach to this avoidance of polyester formation which has been described is to use dilute reaction systems (see U.S. Pat. No. 3,418,363). This process has two commercial disadvantages. First, the isolation of the required ester is complex. Second, unless considerable amounts of alcohol, polyol, and monoester are recycled to the system, the yield of diester produced is small when based on the amounts of alcohol and polyol taken: U.S. Pat. No. 3,418,363 recommends molar ratios of 2 to 10 moles each of alcohol and polyol per mole of dibasic acid.
We have now found that provided a particular catalyst is used, and provided that particular molar ratios of alcohol, polyol, and dibasic acid are used, then hydroxyl group containing unsaturated diesters can be prepared in a largely monomeric state by a simple process. The monomeric nature of the product has been established in two separate ways. First, by instrumental analysis techniques, particularly NMR. Second, by successful copolymer preparation both in solution and in an aqueous emulsion.